1. You enter Dark when the Earth is destroyed, and at various other points throughout the game.
2. Beginning with the fourth time that you read "There's nothing you can taste..." (or some paraphrase of that paragraph), one of your five senses will disappear.
3. At this point, you should type the sense that no longer appears, such as SMELL or SEE.
4. The first time you enter Dark, smell will go away. See the second question in this section.